Electrolyte for electrolytic capacitor

ABSTRACT

An electrolyte for an electrolytic capacitor is disclosed containing an alkyl quarternary ammonium salt of phenol or nitrophenols as solute within an aprotic solvent. The electrolyte exhibits high conductivity while eliminating the drawbacks associated with the presence of water.

FIELD OF INVENTION

An electrolyte for electrolytic capacitor containing within an aprotic solvent alkyl quarternary ammonium salts of phenol or nitrophenols as solute.

An electrolyte for electrolytic capacitor according to the invention may conveniently be prepared by adding one equivalent amount of phenol or nitrophenols to a methylene chloride solution of alkyl ammonium hydrogen-sulfate for reaction with subsequent addition of two equivalent amounts of alkali hydroxide. A deposit is removed from the reaction product with subsequent vacuum drying to obtain an anhydrous salt and then dissolving said anhydrous salt to an aprotic solvent until a desired conductivity is obtained, for example, as an approximately 20% by weight solution.

BACKGROUND OF THE INVENTION

An electrolytic capacitor is composed by using an anode electrode of the so-called valve metal such as aluminium or tantalum, said metal being caused to form an insulating oxide layer on its surface, said insulating oxide layer acting as a dielectric layer, contacting said insulating oxide layer with an electrolyte layer, arranging a current collector electrode generally referred to as cathode.

The electrolyte for the electrolytic capacitor, as above mentioned, contacts directly with the dielectric layer and acts as a true cathode, that is, the electrolyte lies between the dielectric layer of the electrolytic capacitor and the current collector electrode and hence its reistance is inserted in series for the electrolytic capacitor. Therefore, the characteristic of the electrolyte results in an important factor controlling the characteristic of the electrolytic capacitor. For example, if a conductivity of an electrolyte is lower, an equivalent series resistance within an electrolytic capacitor is increased and hence a high-frequency characteristic and a loss characteristic may be lowered.

That is the reason of the demand for high conductivity electrolyte and as such a high conductivity electrolyte, a solution of an organic acid or its salt, such as adipic acid in glycols, such as ethylene glycol or alcohols has been used.

The latest expansion of demands of electrolytic capacitors having higher electric characteristics has not been satisfied with the conductivity of the presently available electrolyte. Especially in the case of the presently available electrolyte, when a desired conductivity is not obtained or a low solubility solute is used, the conductivity has been improved by intentionally adding water.

However, in the latest utilizing condition of the electrolytic capacitor required for use at above 100° C. and for many hours, the presence of water causes a dielectric layer to be worsened, an internal vapor pressure of the electrolytic capacitor to be increased and hence the life of the electrolytic capacitor is worsened because of a breakage of a seal part or a volatilization of an electrolyte. Therefore, the presently available electrolytic capacitor has had as a drawback the inability to hold a stable characteristic over a long term. To solve the matter, it is known that an ammonium or a primary, secondary or tertiary amine salt of an organic acid is used as a solute in an aprotic solvent as disclosed in the Japanese patent publication Nos. 55-3989, 57-56763, 58-32769 and the U.S. Pat. No. 4,117,531.

Hitherto, it has been known that ammonium or primary, secondary or tertiary amine salts of phenol or nitrophenols are difficult to form in a polar solvent such as water owing to their weak acidities though the acidities of nitrophenols may be increased by the increase of nitro-groups. However, phenol or nitrophenols can form the salts of ammonium or primary, secondary or tertiary amine in an aprotic solvent, however, such salts are practically insoluble in the aprotic solvent and hence such a solution has substantially no conductivity or an extremely low conductivity. Now, it has been discovered that alkyl quaternary ammonium salts of phenol or nitrophenols have a considerable solubility in an aprotic solvent and the solution has an extremely high conductivity.

SUMMARY OF THE INVENTION

Thus, a principal object of the invention is to provide an electrolyte of substantially non water series high conductivity with use of the aprotic solvent.

An electrolyte for electrolytic capacitor according to the invention is characterized by containing alkyl quaternary ammonium salts of phenol or nitrophenols in the aprotic solvent as solute.

PREFERRED EMBODIMENTS OF THE INVENTION

The alkyl ammonium salts of phenol or nitrophenols according to the invention is represented by the following general formula, ##STR1## in which n represents an integer of 0 to 3, and A is alkyl ammonium containing 1 to 4 alkyl groups having 1 to 6 carbon atoms.

The nitrophenols to be preferably used in the invention may be selected from the group of o-nitrophenol, m-nitrophenol, p-nitrophenol, 2,3-dinitrophenol, 2,4-dinitrophenol, 2,5-dinitrophenol, 2,6-dinitrophenol, 3,4-dinitrophenol, 3,5-dinitrophenol or 2,4,6-trinitrophenol.

The alkyl ammonium to be preferably used in the invention may be selected from the following but not limited thereto:

tetramethylammonium, tetraethylammonium, tetrabutylammonium, trimethylammonium, dihexylammonium, hexylammonium.

The aprotic solvent to be used in the invention may be selected from the following but not limited thereto:

(1) Amide system solvent:

N-methylformamide, N,N-dimethylformamide, N-ethylformamide, N,N-diethylformamide, N-methylacetamide, N,N-dimethylacetamide, N-ethylacetamide, N,N-diethylacetamide, hexamethylphosphorinamide

(2) Oxide compounds:

dimethyl sulfoxide

(3) Nitrile compounds:

acetonitrile

(4) Cyclic esters, cyclic amides:

γ-butyrolactone, N-methyl-2-pyrrolidone, ethylene carbonate, propylene carbonate.

An electrolyte for electrolytic capacitor according to the invention may conveniently be prepared by adding one equivalent amount of phenol or nitrophenols to a methylene chloride solution of alkyl ammonium hydrogen-sulfate for reaction with subsequent addition of two equivalent amounts of alkali hydroxide. A deposit is removed from the reaction product with subsequent vacuum drying to obtain an anhydrous salt and then dissolving said anhydrous salt in an aprotic solvent unitl a desired conductivity is obtained, for example, as an approximately 10% to 20% by weight solution.

(EXAMPLES 1 TO 11)

An electrolyte for electrolytic capacitor according to the invention wll be exemplified in the following with 20% by weight solution for alkyl ammonium salts of phenol and various nitrophenols with respect to the conductivity as shown in Table 1.

Further, as a comparative example the conventional standard electrolytes composed of 78% by weight of ethylene glycol, 10% by weight of water and 12% by weight of ammonium adipate are shown also in Table 1.

                  TABLE 1                                                          ______________________________________                                         Ex-                       Weight  Conductivity                                 ample Formulation of electrolyte                                                                         (%)     (ms/cm)                                      ______________________________________                                         1     tetramethylammonium 20      11.9                                               phenolate                                                                       ##STR2##                                                                      N,Ndimethylformamide                                                                               80                                                   2     tetramethylammonium 20      8.3                                                o-nitrophenolate                                                                ##STR3##                                                                      Nmethylpyrrolidone  80                                                   3     tetraethylammonium  20      7.1                                                m-nitrophenolate                                                                ##STR4##                                                                      propylene carbonate 80                                                   4     tetrabutylammonium  20      20.0                                               p-nitrophenolate                                                                ##STR5##                                                                      acetonitrile        80                                                   5     tetramethylammonium 20      9.5                                                2,3-dinitrophenolate                                                            ##STR6##                                                                      γ-butyrolactone                                                                              80                                                   6     tetraethylammonium  20      8.7                                                2,4-dinitrophenolate                                                            ##STR7##                                                                      dimethyl sulfoxide  80                                                   7     tetrabutylammonium  20      12.5                                               2,5-dinitrophenolate                                                            ##STR8##                                                                      N,Ndimethylformamide                                                                               80                                                   8     tetramethylammonium 20      9.0                                                2,6-dinitrophenolate                                                            ##STR9##                                                                      N,Ndimethylacetoamide                                                                              80                                                   9     tetraethylammonium  20      12.0                                               3,4-dinitrophenolate                                                            ##STR10##                                                                     Nmethylformamide    80                                                   10    tetrabutylammonium  20      17.8                                               3,5-dinitrophenolate                                                            ##STR11##                                                                     acetonitrile        80                                                   11    tetraethylammonium  20      10.0                                               picrate                                                                         ##STR12##                                                                     Nmethylformamide    80                                                   Ref.  ethylene glycol     78                                                         water               10      6.7                                                ammonium adipate    12                                                   ______________________________________                                    

As understandable from the result above, the electrolyte of the present invention has higher conductivty than that of the conventional electrolyte.

Further, a series of electrolytic capacitors for a comparative test are prepared with the electrolytes of the examples and each of the capacitor characteristics is compared.

The electrolytic capacitor for the test is prepared by superimposing three layers a cathode and an anode of aluminium foil and a separator paper between them to form a cylindrical capacitor element. The capacitor element is impregnated with each of electrolytes of the examples and then is introduced into an external case and sealed respectively.

Each electrolytic capacitor consists of the same dimentional capacitor element having 16 V rated voltage and 180 μF rated capacity.

The actual high-temperature load test obtained at 16 WV 180 micro F for electrolytes exemplified in examples 1 to 11 and the comparative example are shown at the mean value of 10 load tests in Table 2.

                  TABLE 2                                                          ______________________________________                                         Initial Value       110 deg. C. 1000 hours.                                                           Leakage            Leakage                                     Capacity tan    Current                                                                               Capacity                                                                              tan  Current                              Example                                                                               μF    δ                                                                               μA  μF  δ                                                                             μA                                ______________________________________                                         1      179      0.066  0.60   170    0.078                                                                               0.51                                 2      177      0.077  0.63   171    0.095                                                                               0.56                                 3      177      0.080  0.66   165    0.108                                                                               0.59                                 4      182      0.044  0.54   164    0.058                                                                               0.47                                 5      178      0.073  0.58   173    0.087                                                                               0.51                                 6      177      0.075  0.63   172    0.094                                                                               0.56                                 7      179      0.065  0.61   171    0.077                                                                               0.52                                 8      177      0.075  0.65   168    0.098                                                                               0.60                                 9      179      0.066  0.61   174    0.073                                                                               0.51                                 10     181      0.050  0.55   162    0.066                                                                               0.48                                 11     178      0.072  0.58   173    0.079                                                                               0.53                                 Ref.   176      0.085  0.76   152    0.127                                                                               0.73                                 ______________________________________                                    

As understandable from the result above, the electrolytic capacitor using the electrolyte of the present invention has the lower capacitor losses or tan δ as compared with that of the conventional electrolyte because of the higher conductivity of the electrolyte of the present invention.

Furthermore, as the result of the essentially non-aqueous system, the electrolytic capacitor using the electrolyte of the present invention shows no abnormal exterior or decrease of the electrostatic capacity due to the increase of the internal pressure and the extremely few changes of the characteristic values in comparison of the initial value with the value after the load test.

The electrolyte according to the invention may provide the electrolytic capacitor for the power device such as the switching regulator used at high frequency and requiring high efficiency and for various electrical machineries and apparatuses used at high temperature for a long term because of the maintenances of the lower capacitor losses and the stable characteristic used at high temperature for a long term. 

What is claimed is:
 1. An electrolyte for electrolytic capacitor comprising an aprotic solvent and alkyl ammonium salts of phenol or nitrophenols of general formula, ##STR13## in which n represents an integer of 0 to 3, and A is alkyl ammonium containing 1 to 4 alkyl groups having 1 to 6 carbon atoms.
 2. An electrolyte for electrolytic capacitor according to claim 1, wherein aprotic solvent is selected from the group of N-methylformamide, N,N-dimethylformamide, N-ethylformamide, N,N-diethylformamide, N-methylacetamide, N,N-dimethylacetamide, N-ethylacetamide, N,N-diethylacetamide, γ-butyrolactone, N-methyl-2-pyrrolidone, ethylene carbonate, propylene carbonate, dimethyl sulfoxide, acetonitrile or the mixture thereof.
 3. An electrolyte for electrolytic capacitor according to claim 1, wherein nitrophenols are selected from the group of o-nitrophenol, m-nitrophenol, p-nitrophenol, 2,3-dinitrophenol, 2,4-dinitrophenol, 2,5-dinitrophenol, 2,6-dinitrophenol, 3,4-dinitrophenol, 3,5-dinitrophenol or 2,4,6-trinitrophenol.
 4. An electrolyte for electrolytic capacitor according to claim 1, wherein alkyl ammonium is selected from the group of tetramethylammonium, tetraethylammonium, tetrabutylammonium, trimethylammonium, dihexylammonium, hexylammonium. 